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Does harvest of the European grayling, Thymallus thymallus (Actinopterygii: Salmoniformes: Salmonidae), change over time with different intensity of fish stocking and fishing effort?


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Background. The European grayling, Thymallus thymallus (Linnaeus, 1758), is a fish species of high value in recreational fishing. The monitoring of changes in grayling populations is a high priority in fisheries. Data on the harvest of recreational anglers can potentially serve as an easy and inexpensive way to monitor changes in fish populations. This study aimed to assess spatio-temporal trends in catches of grayling in a larger geographical area. Materials and methods. This study analysed harvest rates of grayling by recreational anglers on 241 fishing grounds, in the Czech Republic, within 1986–2015 (30 years). Data from individual angling logbooks were used. The data were collected by individual anglers and processed by the Czech Fishing Union (Český rybářský svaz). Results. Over the period of 30 years, Czech anglers harvested a total of 9 928 grayling specimens weighing altogether 3 357 kg. Within the period surveyed, both parameters (the grayling biomass harvested and the representation of grayling in overall fish harvest) decreased to 10% of the initial values. The percentage of fishing grounds with a harvest of grayling decreased to 30% of the initial values. Harvest per effort decreased to 20% of the initial values over 11 years. There was only a weak correlation between fish stocking and fish harvest. There was a negative relation between the number of angler fishing visits with both catch (fish number) and yield (biomass) of grayling. The harvest was positively correlated with fishing effort. The mean size of harvested grayling remained constant (~0.35 kg) over 30 years. Conclusion. Harvest of grayling significantly declined over the last three decades, implying that increased effort in conservation of grayling is necessary. Future studies should focus on monitoring of the remaining self-reproducing grayling populations.
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Roman LYACH and Jiri REMR
Institute for evaluations and social analyses, Prague, the Czech Republic
Lyach R., Remr J. 2020. Does harvest of the European grayling, Thymallus thymallus (Actinopterygii:
Salmoniformes: Salmonidae), change over time with different intensity of sh stocking and shing
effort? Acta Ichthyol. Piscat. 50 (1): 53–62.
Background. The European grayling, Thymallus thymallus (Linnaeus, 1758), is a sh species of high value in
recreational shing. The monitoring of changes in grayling populations is a high priority in sheries. Data on the
harvest of recreational anglers can potentially serve as an easy and inexpensive way to monitor changes in sh
populations. This study aimed to assess spatio-temporal trends in catches of grayling in a larger geographical area.
Materials and methods. This study analysed harvest rates of grayling by recreational anglers on 241 shing
grounds, in the Czech Republic, within 1986–2015 (30 years). Data from individual angling logbooks were used.
The data were collected by individual anglers and processed by the Czech Fishing Union (Český rybářský svaz).
Results. Over the period of 30 years, Czech anglers harvested a total of 9 928 grayling specimens weighing
altogether 3 357 kg. Within the period surveyed, both parameters (the grayling biomass harvested and the
representation of grayling in overall sh harvest) decreased to 10% of the initial values. The percentage of shing
grounds with a harvest of grayling decreased to 30% of the initial values. Harvest per effort decreased to 20%
of the initial values over 11 years. There was only a weak correlation between sh stocking and sh harvest.
There was a negative relation between the number of angler shing visits with both catch (sh number) and yield
(biomass) of grayling. The harvest was positively correlated with shing effort. The mean size of harvested
grayling remained constant (~0.35 kg) over 30 years.
Conclusion. Harvest of grayling signicantly declined over the last three decades, implying that increased
effort in conservation of grayling is necessary. Future studies should focus on monitoring of the remaining self-
reproducing grayling populations.
Keywords: angling diaries, catch per unit effort, sheries management, population dynamics, salmonids
* Correspondence: Dr Roman Lyach, Institute for Evaluations and Social Analyses/Institut evaluací a sociálních analýz, 186 00 Prague/Praha, the Czech Republic, phone:
+ 420 737 242 256, e-mail: (RL),, (JR), ORCID: (RL) 0000-0001-7783-8256.
DOI: 10.3750/AIEP/02643
In central Europe, the European grayling, Thymallus
thymallus (Linnaeus, 1758), is a sh species of high
value in recreational shing, commercial shing, and
species conservation. Grayling is a native sh species
that used to be common in streams and smaller rivers
located under mountain ranges (Persat 1996). However,
anglers, sheries managers, and environmentalists claim
that populations of grayling have been steadily decreasing
in central Europe over the last 20–30 years (Gum et al.
2009, Weiss et al. 2013, Mueller et al. 2018). Recently,
grayling has become one of the most threatened inland
freshwater sh species in central Europe. By studying
the effect of both natural and human-induced effects on
grayling populations, other authors discovered that the
main reasons for the decrease in grayling populations
are droughts, climate change, predation from sh-eating
birds and mammals, shing pressure, river damming and
straightening, loss of spawning substrates, land-use, and
pollution (Northcote 1995, Persat 1996, Uiblein et al.
2001, Gum et al. 2003, Duftner et al. 2005, Sternecker et
al. 2014, Geist and Hawkins 2016, Bierschenk et al. 2019).
By studying human–sh interactions, previous studies
have also found that the interaction between recreational
sheries and grayling is one of the most important drivers
of grayling populations (Duftner et al. 2005, Näslund et
al. 2005, 2010).
Data from angling logbooks can be used as proxy
data for changes in abundances of sh populations
(Sztramko et al. 1991, , Gudbergsson 2004, Jayasynghe
et al. 2006, Mosindy and Duffy 2007, Skov et al. 2017,
Kerr unpublished*). The results of the studies listed above
suggest that a change in harvest rate potentially suggests
a change in abundance in the ecosystem. In addition, data
from angling logbooks were previously used by other
authors to monitor sh abundances and populations,
Lyach and Remr
water quality, sh sizes, effects of water damming on sh
populations, and changes in water temperatures (Cowx
and Broughton 1986, Binet 1997, Lorenzen et al. 1998,
Draštík et al. 2004, Jayasynghe et al. 2006, Younk and
Perreira 2007, Zeeberg et al. 2008, Gerdeaux and Janjua
2009). The interaction between anglers and grayling is
crucial in the conservation of grayling. However, there
are only a few studies that describe the angler–grayling
interaction (Näslund et al. 2005, 2010). In addition, those
studies describe catches of grayling on small spatio-
temporal scale. No study describes the angler–grayling
interaction on a larger spatio-temporal scale in a larger
geographical area.
This study aimed to describe changes in the catch
(sh number) and yield (biomass) of grayling on a large
spatio-temporal scale (241 studied shing grounds, 30
years of data) in the Czech Republic. We expected that
the harvest of grayling was decreasing, mostly because
anglers, sheries managers, and environmentalists claim
that grayling populations have been declining. It was
hypothesized that both catch and yield would follow
and mirror this trend. Another aim of this study was to
describe changes in the number of shing grounds with
actual catches of grayling. We expected that smaller
grayling populations would perish over time, and that
human-induced effects would lead to a decreased number
of streams where self-sustainable grayling populations
exist. Another aim was to assess changes in the size of
caught grayling over time. We expected that anglers would
be catching smaller sh every year, mostly because the
angling and predation pressure on grayling populations
seems to be increasing. The last aim was to assess the
effect of sh stocking on the sh harvest. We expected
that shing grounds with higher stocking rates would also
display higher harvest rates.
Study area. This study was carried out in the regions of
Prague (50°N, 014.5°E) and the Central Bohemian Region
(Středočeský kraj) (49.5°–50.5°N, 013.5°–015.5°E),
Czech Republic, central Europe (for map of the study area
see Lyach and Čech 2018a). Both regions together cover
an area of 11 500 km2. The region of Prague has mostly
urban character while the region of Central Bohemia has
a mostly rural character. The study area is dominated by
the rivers Elbe and Vltava. Both rivers belong to the upper
Elbe River basin. All rivers in the study area belong to
the North Sea drainage area. Studied shing grounds are
situated in lowlands of an altitude of 200–600 m above sea
level. Waters in the study areas are mostly mesotrophic
and eutrophic with biomass of 150–300 kg of sh per ha
(Lyach and Čech 2017b, 2018a, 2018b). The study area
includes salmonid streams (smaller streams, mostly <
10 m wide, usually dominated by salmonids) and non-
salmonid rivers (wider streams and rivers, usually 10–300
m wide, dominated by cyprinids or percids). Studied
rivers and streams are mostly at their carrying capacity
due to natural sh reproduction and intensive sh stocking
(Závorka et al. 2013). Grayling is a native sh species in
the study area.
Recreational shing in the Czech Republic. Recreational
shing in the Czech Republic is organized by the Czech
Fishing Union (Český rybářský svaz). The Union is the
principal authority in recreational shing in the Czech
Republic and is centralized for the whole country. Each
angler has to carry an angling logbook with him/her at all
times during shing. When an angler catches and keeps a
sh, he/she is obliged to write down the catch (identied
sh species, size of the sh in TL in cm, date of the catch,
and ID/name of the shing site). Filled logbooks are then
collected in January of the following year by the Czech
Fishing Union. Only anglers who submit their old lled
angling logbook will receive a new angling logbook for
the next year. Errors in lling of angling logbooks may
results into conscation of shing equipment, harvested
sh, or shing permit. Proper usage of angling logbooks
is checked in the eld by angling guards. Only killed
(harvested) sh are recorded in individual angling
logbooks. Fish that are undersized, caught during the
closed season, or otherwise released are not recorded in
logbooks. For a detailed description of recreational shing
in the Czech Republic see Lyach and Čech (2018a, 2018b).
Angling rules for grayling. Grayling, Thymallus
thymallus, is an important sh species in recreational
shing in the Czech Republic. The bag limit for salmonids
is either two sh or 7 kg of sh per angler per day,
whichever comes rst. Within 1986–2015 the minimum
legal angling size for grayling was 30 cm (TL, total
length). Any grayling that does not reach this size has to
be returned back to the water without any unnecessary
delay. All harvested graylings must be noted in individual
angling logbooks, including the date of catch, the weight
of sh, and the ID of the shing ground.
Grayling stocking. Annual stocking of grayling is
common and traditional in the study area. Most stocking is
performed on smaller salmonid streams and rivers (<10–
20 m wide) outside the main rivers. Grayling is mostly
stocked as 1–2 year sh (5–10 cm TL). Fish are usually
stocked in hundreds or thousands per stream. The main
goal of the sh stocking is to support wild populations.
Before sh stocking occurs, all stocked sh are weighed
together (in one bag) to the nearest 100 g. The number
of stocked sh is then estimated from the overall weight
by applying length–weight equations of the specic sh
species. The length–weight equations are based on data
from catches of a larger amount (at least 100) of sh that
were caught in the study area by sheries managers. Fish
stocking is performed by local sheries managers.
Data sources. Data from annual summaries of all
collected angling logbooks were used for this study. This
data originated from angling logbooks that were collected
from individual anglers. Fishing grounds are dened as
stream and river stretches where recreational shing can be
legally conducted. The selected shing grounds covered an
* Kerr S.J. 1996. A summary of Muskies Canada Inc. angler log information, 1979–1994. Technical Report TR-011. Ontario Ministry of Natural Resources, Kemptville
ON, Canada.
Harvest of grayling versus sheries management 55
area of 125 km2. This data was originally collected by the
Czech Fishing Union and later processed by the authors
of this study. Data from 241 shing grounds collected
within 2005–2015 (11 years) were used to analyse catch
and yield per shing effort (data on shing effort were
available only from the year 2005 onwards). For that
reason, the harvest of grayling over the years 1986–2004
was not related to shing effort in the analyses. Data from
years 2016 and 2017 were not used because the legislative
rules in recreational shing signicantly changed since
2016 (minimum legal angling size of grayling was
increased from 30 cm to 40 cm TL, total length). In the
rest of the analyses, data from 241 shing grounds within
1986–2015 (30 years) were used. A similar dataset was
previously used for scientic purposes (Humpl et al. 2009,
Jankovský et al. 2011, Boukal et al. 2012, Lyach and Čech
2017a, 2018a, 2018b).
Biometric data. This study assessed the overall catch
(number of sh individuals killed) and yield (total weight/
biomass of all sh killed), catch and yield per one hectare
of shing grounds, catch and yield per effort (one shing
visit), the representation of grayling in the overall sh
harvest, sh body sizes (medium body weight), the
percentage of shing grounds with and without harvested
grayling, catch per stocked sh per hectare, and yield per
stocked biomass per hectare. To estimate the effect of sh
stocking on sh catch, data were used on sh stocking
from 3–5 years before the sh were caught. The mean
value of three consecutive years was used in the analysis.
Data on sh stocking from 0–2 years before the catch
were not included because stocked sh were small (10 cm
TL) and unlikely to grow to legal angling size (30 cm)
over two years. Data on sh stocking that were six years
or older were also not included, mainly for two reasons:
(1) the usual lifespan of grayling is 5–6 years maximum,
and (2) stocked sh usually display high mortality due to
stocking stress, predation, angling, and inability to adapt to
natural conditions. Stocked graylings were very unlikely
to survive for six years in the study area. Therefore, the
effect of stocking on catch and yield was calculated based
on data collected within 1991–2015.
Statistical analyses. The statistical programme R (R
i386 3.4.1.; R Development Core Team 2017) was used
for statistical testing. The package for generalized linear
mixed models (GLMM) was used to t the models
(Hadeld 2010). The function lmer in the package lme4
(version 0.99937542; Bates et al. 2015) was used to
calculate R-squared values (Nakagawa et al. 2013). In the
models, catch (sh number), yield (biomass), and body
weight of sh were used as explained variables. The
year, the intensity of sh stocking, and shing visits were
used as explanatory xed variables. The shing ground
variable was used as a random factor. One shing ground
was used as one sample in the analysis. Gamma error
distribution with log link function was used in the models
that described changes over time. The basic equation for
models was
Catch ~ shery + year
In other models, the catch was replaced with yield or
size, and (1|shery) was removed from the analysis in
the case when the model described the number of shing
grounds with and without catches of grayling. All shing
grounds were used in the analysis of sh harvest. Only
shing grounds with non-zero catches of sh (any species)
were used in the analysis of the representation of grayling
in overall catch and yield. Only shing grounds with non-
zero catches of grayling were used in the analysis of size
(body weight) of caught grayling. Only shing grounds
with non-zero stocked grayling were used in the analysis
of the effect of sh stocking on catch and yield. The
minimum probability level of P = 0.05 was accepted for
all the statistical tests, and all statistical tests were two-
tailed. Bonferroni correction was applied when multiple
groups were compared in statistical analysis. The results
presented in the Table 1 are derived from models in R
while the gures were drawn in MS Excel. The method
described above was previously used to analyse similar
data sets on sh harvest (Humpl et al. 2009, Jankovský
et al. 2011, Boukal et al. 2012, Lyach and Čech 2017a,
2018a, 2018b, Lyach and Remr 2019a, 2019b).
Overall summary. Within 1986–2015 (30 years), anglers
caught altogether 9 928 graylings of the total weight of
3 357 kg. In comparison, over 30 years, anglers caught
altogether 7 715 156 sh (of different species) of the
total weight of 11 512.87 t. Within 2005–2015 (11 years),
anglers visited selected shing grounds 5 739 535 times
and caught 1 320 graylings of the total weight of 436 kg.
In comparison, over 11 years, anglers caught altogether
2 234 110 sh (all species) of the total weight of 4 385
t. Anglers visited one hectare of studied shing grounds
238 times, on average, and harvested 0.0096 graylings of
the biomass of 0.0032 kg per hectare of shing grounds
annually. Fisheries managers stocked 1.44 graylings of
the biomass of 0.03 kg per hectare of shing grounds
annually. The results of all used statistical models are
listed in Table 1.
Catch and yield of grayling. Both catch and yield of
grayling decreased to 10% of the initial values over the
course of 30 years (Figs. 1A, 1B). Anglers were catching
0.6 sh and 0.2 kg of sh per hectare of shing grounds
in 1986. However, catch and yield decreased to only 0.07
sh and 0.02 kg of sh per hectare of shing grounds in
the year 2015. The model explained 16% and 5% of the
variability in catch and yield, respectively.
Catch and yield per shing visit. Anglers were catching
fewer grayling per shing effort (shing visit) every year.
Both catch and yield per shing visit decreased to 20%–
25% of the initial values over 11 years (Figs. 2A, 2B).
Anglers caught 0.0003 sh and 0.0001 kg of sh per visit
in 2015. After 11 years, both catch and yield per visit
dropped to 0.00007 sh and 0.00003 kg of sh per shing
visit, respectively, in 2015. The model explained 11% and
12% of the variability in catch and yield, respectively.
Both catch and yield were positively correlated to the
shing effort (intensity of shing visits). Fishing grounds
Lyach and Remr
with higher visit rates also displayed higher catch and yield
of grayling. However, shing grounds with the highest
visit rate (35 000–100 000 visits per year) displayed zero
catches of grayling (Figs. 2C, 2D). The positive effect of
shing effort on catch and yield was mostly observed on
shing grounds with lower visit rates (10–10 000 visits
per year). For larger shing grounds, there was a negative
relation between the number of angler shing visits with
both catch and yield of grayling.
Representation in overall catch and yield. The
representation of grayling in the overall catch and yield
of all sh decreased to 20% and 10% of the initial value
(in catch and yield, respectively) over the course of 30
years. The representation of grayling decreased from
0.24% to 0.05% and from 0.08% to 0.008% in catch and
yield, respectively (Figs. 3A, 3B). The model explained
5% of the variability in the representation in both catch
and yield.
Harvest in relation to sh stocking. There was only a
weak correlation between sh stocking and sh harvest.
Higher intensity of sh stocking did not lead to signicantly
higher rates in the sh harvest. Fisheries managers
stocked 5–1000 sh with a total weight of 0.1–27 kg per
one hectare of shing grounds, however, several shing
grounds with high intensity of sh stocking displayed zero
harvested grayling. Inversely, several shing grounds with
low intensity of sh stocking displayed relatively high
harvest rates (considering that overall harvest of grayling
was very low in general).
Fishing grounds with catches. Anglers were catching
grayling on a lower number of shing grounds every
year. The percentage of shing grounds with one or more
harvested grayling was decreasing over time. The number
of shing grounds with catches of grayling decreased to
30% of the initial value (from 12.5% to 3.7%) over 30 years
(Fig. 4A). The model explained 18% of the variability in
the percentage of shing grounds with sh catches.
Size of caught sh. Anglers were catching grayling of
comparable size (body weight) every year. The size of
Table 1
Changes of basic metrics in recreational shing over time with different sheries management (data are for catches
of grayling Thymallus thymallus by recreational anglers in the Czech Republic within 1986–2015)
Dependent variable Explanatory variable Intercept ± SD Slope ± SD P-value Var (RE) R2DF
Catch × ha–1 year 0.35 ± 0.14 –0.048 ± 0.0030 <0.001 2.8200 0.1600 2 681
Yield × ha–1 year 0.19 ± 0.11 –0.016 ± 0.0020 <0.001 1.5940 0.0500 2 681
Catch × shing visit–1 year 4.22 ± 1.14 –0.002 ± 0.0009 <0.001 0.0004 0.1100 2 681
Yield × shing visit–1 year 2.02 ± 0.75 –0.001 ± 0.0003 0.007 0.0002 0.1200 2 681
Catch × ha–1 shing visit 0.023 ± 0.071 0.00004 ± 0.000010 0.002 0.0048 0.04 2 681
Yield × ha–1 shing visit 0.012 ± 0.021 0.00020 ± 0.000004 0.002 0.0039 0.04 2 681
% in overall catch year 27.24 ± 4.37 –0.013 ± 0.0020 <0.001 0.0050 0.0500 2 681
% in overall yield year 24.87 ± 4.49 –0.012 ± 0.0022 <0.001 0.0180 0.0500 2 681
Catch × visit–1 stocked sh n × ha–1 0.00016 ± 0.00007 0.00008 ± 0.00003 0.26 0.00003 0.008 566
Yield × visit–1 stocked b. × ha–1 0.00009 ± 0.00003 0.00003 ± 0.00001 0.21 0.00001 0.003 566
N of sites with catches year 6.78 ± 2.78 –0.003 ± 0.0010 0.016 NA 0.1800 2 681
Mean body weight year –23.82 ± 2.98 0.012 ± 0.0014 0.410 0.0030 0.1800 164
SD = standard deviation, var (RE) = variance for random effect, DF = degrees of freedom, NA = not applicable; N = number; stocked b. =
stocked biomass, stocked sh n = stocked sh number.
Fig. 1. (A) Catch (sh number) and (B) yield (biomass)
of grayling, Thymallus thymallus, per hectare of shing
grounds in the Czech Republic within 1986–2015; the
whiskers represent the standard error of mean
Harvest of grayling versus sheries management 57
caught grayling did not signicantly change over 30 years
(Fig. 4B). The mean size of caught grayling per shing
ground was 0.35 kg and ranged from 0.25 kg to 1.8 kg.
The model explained 18% of the variability in sh size.
Data limitations. The dataset that is derived from
individual angling logbooks provided long-term data
on sh catches on a large number of shing grounds,
however, the data should be used and interpreted with
caution. Fish catches are reported by regular anglers
and not by scientists. Since the data are based on citizen
science, the error in the data is probably a bit higher
when compared to real scientic data. On the other hand,
recreational shing connects regular people to nature,
and, to a certain point, to scientic work. That is a big
advantage in a similar type of research. However, this
dataset has several limitations. Anglers may overestimate
or underestimate the numbers and sizes of caught sh,
disobey shing rules, and incorrectly identify harvested
species. Listed errors are made either unknowingly or on
purpose (Essig and Holliday 1991, Pollock et al. 1994,
Cooke et al. 2000, Bray and Schramm 2001, Mosindy
and Duffy 2007, Lyach and Čech 2017a, 2018a, 2018b).
Fisheries data also do not cover poaching or the catch-
and-release shing strategy. Especially salmonids display
high post-release mortality (Clark 1991, Casselman 2005).
However, the dataset provided data on 30 years of catches
on 240 shing grounds, and the data were collected by
approximately 60 000–80 000 different people during
at least tens of millions of working hours (Lyach and
Čech 2018a, 2018b). If the data were collected by a few
scientists, the bias in the selective collection of the data
would be higher, mostly because every person performs
shing a bit differently. It would also be impossible to
collect data on this strength. This dataset was collected
by approximately 60 000–80 000 people in the eld, and
therefore the bias in data collection should be low.
Fig. 2. (A, B) Catch (sh number) and yield (biomass) of grayling, Thymallus thymallus, per shing visit; (C, D) the
relation between shing visit rates and harvest (catch and yield) in the Czech Republic within 1986–2015; the
whiskers (A and B) represent the standard error of mean
Lyach and Remr
Political changes. Both catch and yield displayed a
signicant and visible change over the years 1989 and
1990. In 1989, the velvet revolution (fall of the communist
regime) took place in Czechoslovakia. The majority of
metrics in recreational shing were increasing until 1989,
and after that, those metrics started to decrease. Fishing
used to be a very popular leisure activity during the
communist era, mostly because regular people were not
allowed to travel to the western capitalist world (Europe,
North America), and the possibilities of travelling to
eastern Europe were very limited as well. Other means
of entertainment were also signicantly limited. People
shed to obtain food, mostly because food supplies were
also limited and often not available. After 1989, the
borders opened and people could travel, participate in a
wide variety of leisure activities, and buy the food that
they wanted. For that reason, the popularity of shing
decreased, and the number of shing visits also decreased.
That could have caused a decrease in catch and yield. The
agricultural management also changed after 1989; the
input of fertilizers into the environment decreased. That
caused a decrease in primary production in most water
ecosystems (Kunzová and Hejcman 2009). Unfortunately,
data on sh harvest before 1986 were not available.
It seems that both catch and yield increased from 1986
through 1989, and it would be interesting to see when
exactly the increasing trend started. In conclusion, it seems
that the fall of the regime was one of the most important
factors in recreational shing.
Catch and yield. Both catch (sh number) and yield
(biomass) are usually linked to the following three
parameters: population changes of sh species in the
environment, popularity of the catch-and-release shing
strategy, and interest in conservation of sh species
(Jayasynghe et al. 2006, Mosindy and Duffy 2007,
Skov et al. 2017). Those three parameters are also
interconnected; anglers are more likely to release rare
and endangered sh species (Arlinghaus et al. 2007,
Fig. 3. (A, B) The percentage representation of grayling, Thymallus thymallus, in the overall catch (sh number) and yield
(biomass) of all sh caught by anglers in the Czech Republic within 1986–2015; (C) the relation between the amount
of stocked sh per hectare of shing grounds and catch per shing visit; (D) the relation between stocked biomass
per hectare of shing grounds and yield per shing visit; the whiskers (A and B) represent the standard error of mean
Harvest of grayling versus sheries management 59
Bartholomew and Bohnsack 2005). Since anglers are
well aware of the poor population status of grayling, it
is possible that decreased harvest was partially caused
by the increasing popularity of catch-and-release shing.
By studying sheries discussion forums on sheries Web
pages, we found that anglers are strongly supporting the
conservation of grayling. Anglers claim that they are
releasing all caught grayling (Authors’ observation).
When the results of this study are combined with
opinions of local anglers and sheries managers, it can
be concluded that this dataset provided good proxy data
on changes in grayling populations in the study area.
Fishing grounds with catches. The number of shing
grounds with reported catches of grayling were relatively
low already 30 years ago, and the number was decreasing
over time. Grayling is a typical inhabitant of streams, and
the majority of streams in the area are not listed as shing
grounds (Czech Fishing Union, unpublished data). Instead,
they are listed as waters that are used for spawning and
breeding purposes (shing is not allowed there). Streams
in the study area are signicantly affected by pollution,
predation from piscivorous predators (otter Lutra lutra,
cormorant Phalacrocorax carbo, heron Ardea cinerea,
mink Mustela vison), shing pressure, and migration
barriers (Adámek and Jurajda 2001, Humpl and Pivnička
2006, Slavík et al. 2012, Závorka et al. 2013, Lyach
and Čech 2017a, Lyach et al. 2018). Another problem
is a shortage of grayling for stocking purposes. There is
usually not enough grayling to spawn, and therefore the
amount of YOY sh and yearlings available for stocking
is very limited (Czech Fishing Union, unpublished data).
Articial rearing of grayling in aquaculture is signicantly
less protable than the rearing of common carp, Cyprinus
carpio Linnaeus, 1758, or rainbow trout, Oncorhynchus
mykiss (Walbaum, 1792) (see Carlstein 1995). Import of
grayling from abroad is not recommended due to genetic
differences in sh populations (Gum et al. 2009).
Fish stocking. The effect of sh stocking on catch and
yield could be different in areas with pristine unpolluted
streams that support native grayling populations.
Especially streams that are situated in the mountains will
likely show higher catches of grayling. Return rates of
grayling in areas with pristine streams could exceed 100%,
mostly because anglers can catch both native and stocked
grayling there. For example, the biomass of harvested
graylings could be higher when compared to the biomass
of stocked graylings. This effect was observed for self-
reproducing sh populations of very abundant sh species
that are harvested by anglers. For example, European
chub, Squalius cephalus (Linnaeus, 1758), displayed
harvested biomass of 50–100 kg in the Berounka River
(Central Bohemia) even though no stocking of chubs
occurred (Czech Fishing Union, unpublished data).
Similarly, European catsh, Silurus glanis Linnaeus, 1758
displayed harvest rates of 8–10 kg per hectare on the same
river even though no stocking of catsh occurred either
(Lyach and Remr 2019c). There are streams with self-
reproducing grayling populations located under mountains
approximately 100 km from the study area (Horká et al.
2015). However, streams with natural grayling spawning
are rare, and this study describes the situation on typical
lowland streams.
Catch and visit rates. Catch per visit was decreasing even
more rapidly than catch per effort. It is mostly because
anglers were visiting shing grounds more frequently
each year, contributing to increased shing pressure
in the area. As determined by Lyach and Čech (2018a),
the shing pressure has been increasing recently. On the
other hand, both catch and yield have been decreasing
(Lyach and Čech 2018a). Another study found that shing
pressure was the highest on smaller streams where most
grayling are caught (Lyach and Čech 2018b). Therefore,
the effect of recreational shing on grayling populations
could be potentially even more important in the future. The
presently reported study also found that the representation
of grayling in the overall catch was decreasing. Since
Lyach and Čech (2018a) found that the overall sh harvest
Fig. 4. (A) The percentage of shing grounds with and
without a catch of grayling, Thymallus thymallus;
(B) the mean body weight of grayling caught by anglers
in the Czech Republic within 1986–2015; the whiskers
(B) represent the standard error of mean
Lyach and Remr
in the study area was, in general, decreasing, the presently
reported study suggests that harvest of grayling has been
decreasing more rapidly when compared to the majority of
other sh species.
Size of caught sh. The size of caught grayling was
constant over time, most likely because anglers are usually
catching sh that are slightly larger than legal angling
size (30 cm TL, total length). According to the length–
weight equations that anglers use to estimate weights of
caught sh, the mean weight of caught grayling (0.35 kg)
represents a 35 cm (TL) specimen. A 30 cm (TL) large
grayling should weigh 0.25 kg.
In conclusion, the dataset clearly shows that catch and
yield of grayling have been decreasing over the last 30
years. The decrease in catch and yield can be most likely
explained by population decrease and the increasing
popularity of catch-and-release strategy. Intensive sh
stocking had no signicant effect on harvest rates of
grayling, suggesting that intensive stocking of graylings
was ineffective. Larger shing grounds displayed low
harvest rates of grayling, suggesting that anglers who
want to harvest graylings should focus on smaller-sized
rivers and streams. The data also suggest that the fall of the
communist regime had a signicant effect on recreational
shing, mostly because the harvest of grayling started
decreasing immediately after the changes in the regime
in 1989. This was likely due to new possibilities to travel
abroad and also a higher supply of other recreational
activities. This study provided yet another proof that
conservation of grayling as a species is necessary, mostly
because grayling is slowly vanishing from streams and
rivers in central Europe. We believe that anglers, sheries
managers, and environmentalists should join forces
with the scientic community to nd a way to conserve
grayling populations. We also conclude that angling
logbooks provided a very useful set of data that can be
used in sheries research. We suggest that future studies
should focus on monitoring of streams that still support
self-reproducing grayling populations. Similar studies
would hopefully help to conserve grayling populations for
future generations.
The Czech Fishing Union (Český rybářský svaz)
(namely Jaroslava Fryšová, Pavel Horáček, and Dušan
Hýbner) provided necessary sheries data. Pavel Vrána,
Martin Čech, Karel Anders, and Robert Arlinghaus
provided helpful insights into recreational shing. Otakar
Ďurďa and Marek Omelka helped with the statistical
analyses. Anglers and angling guards in the Czech
Republic collected data for this study and therefore made
this study possible. This study was nancially supported by
the Charles University Grant Agency (Grant GA UK No.
112 218), by Charles University (Faculty of Science), and
by the Ministry of Education, Youth, and Sports of the
Czech Republic.
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Received: 31 January 2019
Accepted: 11 November 2019
Published electronically: 1 March 2020
... Bonferroni correction was applied in all three models because multiple groups were tested for differences. This method of fisheries data analysis was previously used to analyze fish harvest rates in different research papers [32][33][34][35]. ...
... The angling effort strongly affected the harvest rates of the rheophilic fishes. Previous studies also agreed that the harvest rates of fish are driven by the angling effort [33,35,45,46]. The angling effort is relatively hard to estimate because each angler fishes at a different intensity. ...
Full-text available
The European catfish Silurus glanis (Linnaeus, 1758) is an expanding apex piscivorous predator whose predation may drive fish harvest rates and fish populations. This study aimed to analyze the relationships between intensive catfish stocking/harvesting and harvest rates of putative catfish prey–three rheophilic fish species: vimba bream Vimba vimba, nase Chondrostoma nasus, and barbel Barbus barbus (Linnaeus, 1758). The GAM (generalized additive model) was used to analyze the relationships between the harvest rate and the stocking intensity rate of the catfish and the three rheophilic fish species. The harvest rates and stocking intensity rates were obtained from mandatory angling logbooks collected from 38,000 individual recreational anglers by the Czech Fishing Union on 176 fishing sites over the years 2005–2017 in central Bohemia and Prague (the Czech Republic). Our results show that a higher intensity of catfish stocking and harvesting resulted in a lower harvest rate of rheophilic fishes. Conversely, the stocking rates of rheophilic fishes were not significantly correlated to their harvest rates. In conclusion, a significant negative relationship was found between the harvest rate and the restocking rates of rheophilic fishes and their predator, suggesting that fisheries managers should not perform intensive stocking of both catfish and rheophilic fishes on the same rivers.
... Some examples of recent research have prioritised the use of directly collected weight data over weights converted from lengths for Southern Bluefin Tuna, Thunnus maccoyii in Australia and in the United States' Marine Recreational Information Programme (Atlantic Coastal Cooperative Statistics Program, 2018;Tracey et al., 2020). However, others have used weights imputed from length data that can be reliably gathered in at least Australia, the European Union and North America (Embke et al., 2020;Lyach and Remr, 2020;Ochwada-Doyle et al., 2019). ...
Quantifying recreational fishing harvest by weight is vital for stock assessments and fisheries management. As it is impractical to obtain weights of all fish caught by recreational fisheries, harvest is often calculated from estimated catch in numbers multiplied by an estimate of average weight (usually as an arithmetic mean). The average weight may be derived from measured weights, as well as imputed weights from measured lengths using established length-weight relationships. This study evaluates the impact of uncertainty in lengths and weights from three independent data sources in determining average weights and estimated recreational harvest of four demersal species in Western Australia. Data sources included measured lengths and weights from on-site surveys of boat-based fishers, self-reported lengths from charter-boat logbooks and lengths from biological samples voluntarily donated by recreational fishers for stock assessments. For the latter two data sources, weights were imputed from length measurements. Generalised linear models were used to obtain estimates of standardised average weight and standard errors for each data source, across four years (2011/12, 2013/14, 2015/16 and 2017/18) and three spatial management zones within the West Coast Bioregion of Western Australia (Mid West, Metropolitan and South West). For each species, harvest estimates and 95% confidence intervals were derived from standardised average weights (from aforementioned surveys) and estimated catch in numbers (from off-site surveys). Standardised average weights from GLMs were found to be more precise than arithmetic means and data from the charter-boat logbooks and biological samples generally produced higher harvest estimates. The application of standardised weights from management zones to estimate recreational harvest at the bioregion level reduced the error of estimates. Addressing uncertainty from self-reporting, data sets (charter-boat logbooks and biological samples) and small sample sizes (on-site surveys) can increase confidence in recreational harvest estimates.
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Inland recreational fisheries, found in lakes, rivers, and other landlocked waters, are important to livelihoods, nutrition, leisure, and other societal ecosystem services worldwide. Although recreationally-caught fish are frequently harvested and consumed by fishers, their contribution to food and nutrition has not been adequately quantified due to lack of data, poor monitoring, and under-reporting, especially in developing countries. Beyond limited global harvest estimates, few have explored species-specific harvest patterns, although this variability has implications for fisheries management and food security. Given the continued growth of the recreational fishery sector, understanding inland recreational fish harvest and consumption rates represents a critical knowledge gap. Based on a comprehensive literature search and expert knowledge review, we quantified multiple aspects of global inland recreational fisheries for 81 countries spanning ~192 species. For each country, we assembled recreational fishing participation rate and estimated species-specific harvest and consumption rate. This dataset provides a foundation for future assessments, including understanding nutritional and economic contributions of inland recreational fisheries.
The literature on global trends in recreational fishing, the determinants of participation in recreational fishing, and the social embedding of recreational fishing in the public eye are reviewed across the world. Data support a conceptual life-cycle model of fisheries according to which interest in recreational fishing rises rapidly with economic development before lev-eling off and eventually declining. Participation in recreational angling across the globe varies substantially and is directly related to societal-level developments affecting resources, time, and socialization into fishing. Moreover, culture and the way that fish are historically situated within society appears to affect interest in fishing and the public perception of certain fishing practices. Across the more developed western countries, a sustained shift in public values from anthropocentric to more biocentric viewpoints is documented. This shift puts traditional fisheries management that manages ecosystems for optimal fishing experiences under increasing scrutiny and elevates biodiversity conservation toward a key goal of contemporary fisheries management in many countries. However, while a pro fish welfare discourse can be traced to almost all developed countries covered in this review, this does not mean the recreational activity is threatened or welfare-oriented regulations are widely implemented, with a few exceptions in selected countries. Public surveys conducted in mainly developed countries around the world instead reveal that people generally view recreational fisheries as an acceptable pastime. Major structural changes are occurring in many societies related to immigration, increasing ethnic and cultural diversity of populations , and social value change. Yet, little is known how these changes might affect recreational fishing participation and behavior, and the view of the general public toward fishing in the future. Panel research designs that repeatedly survey the public, and recreational fish-ers, will be needed to track value and participation changes over time, but such designs are rarely implemented in most countries that were reviewed in this work. Data gaps are particularly strong for Africa and large parts of Central and South America as well as Russia and Asia.
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This study aimed to describe the effect of cormorant predation on newly established Atlantic salmon, Salmo salar, population in three nursery streams in the upper Elbe River basin (Czech Republic). Salmon have been annually stocked into the nursery streams since 1998 as part of a salmon reintroduction programme. Salmon parr density in nursery streams was 3-81 fish per 100 m 2. Only thirteen adult salmon were observed in the study area during two years of research. Altogether 912 cormorant pellets were collected, 5482 diagnostic bones were analysed, and 3915 fish were identified in the diet. Cormorant diet was composed of 24 fish species from six families but no salmon were consumed. The salmon stocking programme produces a reasonable amount of smolts but return rates of adults are very low. The cause of low return rates is not cormorant predation on nursery streams but, most likely, a low survival rate on the passage downstream. We suggest that more studies should focus on monitoring of survival and return rates of salmon in the upper River Elbe to ensure that, in the future, the salmon reintroduction programme will be really successful. Cite as: Lyach R., Čech M. 2017. The effect of cormorant predation on newly established Atlantic salmon population. Folia Zoologica 66 (3): 167-174.
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The coefficient of determination R² quantifies the proportion of variance explained by a statistical model and is an important summary statistic of biological interest. However, estimating R² for generalized linear mixed models (GLMMs) remains challenging. We have previously introduced a version of R² that we called for Poisson and binomial GLMMs, but not for other distributional families. Similarly, we earlier discussed how to estimate intra-class correlation coefficients (ICCs) using Poisson and binomial GLMMs. In this paper, we generalize our methods to all other non-Gaussian distributions, in particular to negative binomial and gamma distributions that are commonly used for modelling biological data. While expanding our approach, we highlight two useful concepts for biologists, Jensen's inequality and the delta method, both of which help us in understanding the properties of GLMMs. Jensen's inequality has important implications for biologically meaningful interpretation of GLMMs, whereas the delta method allows a general derivation of variance associated with non-Gaussian distributions. We also discuss some special considerations for binomial GLMMs with binary or proportion data. We illustrate the implementation of our extension by worked examples from the field of ecology and evolution in the R environment. However, our method can be used across disciplines and regardless of statistical environments.
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The data obtained within the framework of a survey of macrozoobenthos and fish communities in Czech streams of the Danube and Elbe river basins were evaluated with respect to the relation between biodiversity and level of water (organic pollution) and/or physical habitat quality (heterogeneity, substrate, riparian vegetation, canalisation). The diversity of macrozoobenthos species was the highest at the water quality corresponding to betamesosaprobity (saprobiological index SI ≈ 2,0) and oligosaprobity (SI ≈ 1,0) in lowland and highland streams respectively, declining both towards lower and higher saprobic (≈ trophic) levels. The response of macrozoobenthos to habitat quality deterioration was less considerable with rising degradation in highland streams than in lowland ones. Fish assemblage followed a similar trend, namely: the highest biodiversity in betamezosaprobity in both lowland and highland streams. In comparison with the assemblage of benthic macro-invertebrates, fish community response was more pronounced both regarding water quality and habitat degradation. Both fish and macrozoobenthos biodiversity were influenced more by water quality than by physical habitat degradation.
In Central Europe, European grayling Thymallus thymallus is an endangered and vanishing fish species with high recreational angling value. For that reason, in January 2016, the minimum legal angling size for grayling was increased from 30 to 40 cm in the Czech Republic. This study evaluated if the increase in minimum angling size had any effect on grayling harvest. Data from 229 fishing sites covering the years 2011-2017 were used in this study. The data originated from individual angling logbooks, collected in the regions of Prague and Central Bohemia, Czech Republic. Over the seven years, anglers visited the studied fishing sites 3.6 million times and harvested 105 000 salmonids. Grayling made up only 0.5 % of the overall salmonid harvest. The fishing restriction caused a decrease in grayling harvest-per-visit. It also decreased the contribution of grayling to the overall harvest as well as the number of fishing sites where anglers successfully harvested graylings. Fish stocking was constant during the study period. Increased minimum angling size led to increased average body weight of harvested fish. In conclusion, the increase in minimum angling size significantly affected fish harvest and composition.
Intensification of catchment land-use and the corresponding habitat degradation pose a threat to freshwater biodiversity and ecosystem health, yet few studies comprehensively quantified the effects of specific land-use variables on fish communities for different catchments within the same climatic region. Herein, we investigated the influence of catchment land use on fish community composition in the headwater areas of the European main river systems Elbe, Danube and Main/Rhine. The analyses comprising 289 streams and rivers in Bavaria, southern Germany, revealed that the influence of urbanization (e.g. ground sealing), potamalisation (impoundment of water courses), and erosion-prone, agricultural land-use types (e.g. root crop, maize) were significantly related to the fish community composition. In addition, multiple stressors were effective indicators and their importance differed between survey-area scales, geographical regions, and stream sizes. The findings suggest that terrestrial effects of land-use and urbanization need to be more strongly considered in the conservation of endangered stream fishes, ideally including combined measures of erosion control, restoration of environmental flows and mitigation of structural degradation.
Freshwater fishes are among the most threatened groups of vertebrates, with 39% of all European fish species facing extinction. Herein, we provide a comprehensive analyses of historical data as well as fish monitoring data from 1989 through 2013 from Bavaria, Germany. The results of this study indicate that the most pronounced species-turnover already had occurred before the 1990s. Severe loss of species (21 out of 69 species lost until 1990s), decrease in spatial distribution (51 species, 27 reduced to < 50% of historical distribution), decrease of abundance, shifts towards potamal species and the establishment of novel communities due to increasing co-lonization with non-native species was evident. Declines were strongest for gravel-spawning species of the hy-porhitral and epipotamal in medium-sized and large rivers (e.g. grayling (Thymallus thymallus), nase (Chondrostoma nasus), barbel (Barbus barbus)), suggesting that effects of increasing water temperatures and increased fine sediment loads probably strongly contribute to the decline of those species. Our results generally confirmed the validity of current conservation status for most species, but also identified species (dace (Leuciscus leuciscus), chub (Squalius cephalus), trout (Salmo trutta), minnow (Phoxinus phoxinus), nase and barbel) and habitats (medium-sized and large rivers) that deserve higher priority in conservation management. More consistent sampling of the same sites over years and a quantitative monitoring of environmental impact factors in appropriate spatial and temporal resolution is crucial to allow a future prioritization in freshwater fish conservation .
Standardised angler diaries could produce useful proxy data for assessing fish population density and size distribution, but few rigorous studies about their utility exist. We use 62 years of angling diary data (1949–2010), from a large mesotrophic lake, to investigate population structure (abundance, mean size and record size) of European perch (Perca fluviatilis L.) in relation to the impact of three commercial fishers with different fishing strategies, pike (Esox lucius L.) predation and temperature. We found that anglers’ harvest rates of perch varied by a factor of 10 over time, indicating large variation in population abundance over decadal time scales. Our statistical analysis revealed that the anglers’ harvest rates of perch were related to pike CPUE (proxy of pike predation), temperature and commercial fishing directly through the harvest of perch and indirectly through the harvest of pike, the top predator of the lake. The size distribution and growth rates of perch caught by anglers also changed substantially during the study period, most likely controlled by density-dependent mechanisms as well as size-selective commercial harvest. The effect of selective harvest on size-structure was stronger than ecological density dependence. We conclude that commercial harvesting may exert strong impacts on the quality of the angling experiences, at least in the studied case. Moreover, our work showcases the value of detailed angler diaries to study and monitor changes in freshwater fish populations, but it also underlines the need for supplementary data on biotic and abiotic factors to reach the full potential of angler diary data.
Les remontées d'eaux froides qui se produisent le long des côtes d'Afrique occidentale sont des lieux de pêche intensive, mais très fluctuante. Les observations recueillies par les navires marchands permettent d'y suivre l'évolution du vent et de la température de surface et de les comparer aux statistiques de pêche depuis 1964. Dans le courant des Canaries, au large du Sahara et de la Mauritanie, les remontées d'eaux sont essentiellement dues à un upwelling induit par le vent. Le lien avec le vent local est beaucoup moins net dans le courant de Guinée, devant la Côte d'Ivoire et le Ghana. Dans le sud du courant des Canaries, deux périodes d'intensification de l'alizé se sont produites vers 1970-1976 et depuis 1986. Chaque fois, des captures de #Sardina pilchardus$ ont été multipliées par trois environ. Lors du premier événement, il s'est produit une légère régression des prises des #Sardinella$, #Trachurus$, #Decapterus$ et #Scomber$, mieux adaptées à la périphérie des upwellings ; lors du second événement, interrompu par un certain réchauffement, les prises de sardinelles ont augmenté, celles de chinchards ont diminué, et celles de maquereaux ont augmenté uniquement lors du réchauffement. Les captures de sardine sont corrélées à la tension de vent parallèle à la côte au cours de l'année (n-2), à l'exception des premiers mois de la vie larvaire. Toute augmentation de vent induit un enrichissement bénéfique à la survie larvaire, sauf pendant les tout premiers mois suivants l'éclosion, où la turbulence et l'advection vers le large entraînent davantage de pertes. Dans la région du courant de Guinée sujette à des remontées saisonnières de la thermocline, un fort accroissement des captures de #Sardinella aurita$ s'est produit depuis le début des années 1980. Cependant on n'observe pas de baisse de la température de surface qui indiquerait une intensification des résurgences... (D'après résumé d'auteur)
The large area over which European grayling, Thymallus thymallus, extends theoretically prevents it from extinction. However, morphometric and genetic studies have proved that this area is in fact made up of a patch of isolated populations, many of which are highly threatened or have already been eradicated by river alterations. Stocking often appears inefficient because of the low quality of the watercourses, and may also have important genetic implications. A loss of genetic specificity of local wild strains may induce a reduction in their fitness. In France several populations seem to be contaminated by allochthonous genes, but, in good environments, wild strains seem to better resist the introgression. Production of offspring in small hatcheries from wild spawners collected in the river to be stocked is promoted to conserve the genetic identity of local strains. Nonetheless, definite protection will not be expected without the rigorous protection of the habitat for each population.